The proposed project is a continuation of earlier research on the development of learning and memory functioning in neonatal and infant mice. Recent research in this laboratory has suggested that postnatal treatments of thyroxine administration and undernutrition respectively accelerate and delay the maturation of memory functions on certain learning tasks but appear to have little effect upon the maturation of learning on these same tasks. Based upon these findings, the present project proposes to examine the effects of these treatments upon: (1) development of passive-avoidance learning and memory; (b) responsivity to anti-cholinergic and anti-serotonergic agents in both learning and non-learning situations; (c) dendritic development in selected brain areas. In addition, the effects of cerebral protein synthesis inhibition upon infant memory will be explored.